API RP 1111 — Pipeline Wall Thickness Assessment

Executive Summary

1. Pressure-Only Checks (API RP 1111)

Independent of bending moment and tension — these set the baseline utilisation.

2. Capacity Limits by Operating Condition

Pure tension and pure bending limits at util = 1.0 for each API RP 1111 operating condition. Tension limits are asymmetric due to hoop stress interaction: positive tension (pipe stretching) is more favourable than compression when net internal pressure is positive.

2a. Pressure Sensitivity — Effect of Internal Pressure

Capacity limits recomputed at baseline, +50%, and +100% internal pressure. Higher hoop stress shrinks all envelopes and increases the asymmetry between positive and negative tension limits.

3. Combined Loading — Von Mises Interaction

The API RP 1111 combined check evaluates the Von Mises equivalent stress: σ_vm = √(σ_L² − σ_L·σ_h + σ_h²) ≤ f_d·SMYS

The contour plot below maps utilisation across the bending moment vs. tension envelope. The bold black contour at util = 1.0 is the acceptance boundary; the dashed contour at 0.8 marks a typical design target.

4. Allowable Bending Envelope

Each curve traces the maximum allowable bending moment as a function of applied tension, at util = 1.0 for that operating condition. The Normal Operating envelope (innermost) is the most conservative; the Survival envelope (outermost) represents the full yield limit.

Envelope Data — Tension vs Allowable Bending Moment

5. Key Points Summary

Utilisation at selected engineering reference points on the M–T plane.

6. Input Data